The invention relates to a process and an apparatus for melt-spinning of synthetic polymers, preferably of the group comprising polyamide-6, polyamide-66 and polyester and their copolymers, to produce filaments. The polymers are melt-spun through orifices in a spinneret having a substantially planar bottom surface, and the filaments so formed are cooled in a cooling zone located below the uppermost blow point by subjecting them to an air current. The filaments are subsequently moistened and prepared, and finally withdrawn at a speed ranging from 600 to 6000 meters/minute.
Such melt-spinning processes are widely used in the industry. The processing speeds may vary within a wide range. While filaments produced at speeds of under 1800 m/min are drawn and perhaps also textured in separate steps, so-called high speed spun yarns, i.e. filaments or yarns produced at withdrawal speeds in excess of 1800 m/min, are usually draw-textured in one simultaneous operation because of the higher spinning orientation occuring in the filament, or they may be used as completely oriented filaments directly in subsequent textile operations.
In melt-spinning, the course of the cooling process of the filament strands between their point of emergence from the spinneret orifices and the winding package has a decisive influence on the properties of the yarn, such as titre, or tex, and dyeability uniformity, tenacity and extension characteristics.
The filaments emerging from the spinneret orifices at a temperature above the melting point must be cooled to a temperature below the adhesion point, i.e. below the glass transition point of the polymer, before they can be brought into contact with mechanical guide elements. During the cooling period, the filaments experience a considerable reduction in diameter until a final value is reached which corresponds to the spin titre or tex. Hamana, in his article entitled "The Course of Filament Formation in Melt-Spinning," Melliand 4 (1969), pp. 382-388, describes how the properties of the filaments change during the period the filaments are in the cooling zone.
Fourne, in his article entitled "Filament Cooling in Melt-Spinning," Chemiefasern/Textilindustrie, April 1978, p. 315, sets forth mathematical relations which point out the influence of the spinning parameters on the uniformity of the filament properties, in particular the titre, or tex, uniformity.
In theory, titre or tex variations diminish as the withdrawal rate increases. However, the withdrawal speed is dependent on a number of other technological considerations such as, for example, plant capacity and cooling capacity, so that it is not available as a freely variable parameter. Rather, experience has shown that, in both low speed spinning as well as high speed spinning, intolerably high titre variations may occur.
It has further been postulated theoretically that titre variations are a function of the sum of twice the thickness of the insulating jacket of the spinning head and the length of the path in the cooling zone where the filaments are subjected to a blown air current. In the instant case, the thickness of the spinning head insulation is the extent by which the spinning head insulation projects in the downward direction from the underside of the spinneret plate. This is a region which is not reachable by the blown air current. However, since the thickness of the insulating jacket of the spinning head is less than the length of the path traversed by the filaments in the air cooling zone, the thickness value of the insulation is considered to be of only secondary influence. Of a much more critical character seems to be the effect of the air current blown at the filaments and the constancy of the blown air current setup.
In practice, definite standard dimensions for the thickness of the spinning head insulation have been established. The reason for providing a sufficient thickness of the insulating jacket is to prevent the spinning block from cooling off and the spinneret temperature from falling below the product temperature because, in such event, the result would be a severe drop in tenacity and extension values of the filaments and, in the extreme case, a melt break of the capillaries.
From Great Britain Pat. No. 903,427, it is known to pass spun filaments of thermoplastic material through a duct of at least 1 meter in length arranged below the spinneret, at a temperature of 10.degree. to 80.degree. C. below the melting point, in order to obtain good textural yarn characteristics.
U.S. Pat. No. 4,134,882 discloses a high speed spinning process for polyester filaments at speeds of, or in excess of, 5000 m/min. Attention is called to the formation of a so-called core-skin structure of the capillaries as a result of intensive cooling of the filaments. This phenomenon is said to occur especially as the spinning speed increases. The ensuing structural inhomogeneities lead to poor processing properties in subsequent textile operations because of broken or breaking filaments. To avoid this effect, the state of the art proposes the use of a protective duct of a length of 70 to 100 mm to delay the cooling of the filaments. However, it has been found that such a measure will not produce satisfactory filament characteristics.